From the prior art there are rotary encoders known which provide incremental output signals as well as coarse absolute output signals, e.g. commutation output signals. Commutation output signals are necessary in brushless motors where commutation is performed by electronically steering drive current in the motor to the appropriate winding. For this purpose the absolute rotor position has to be determined with a certain accuracy.
There are different ways known from the prior art how to generate the commutation output signals. In a typical method, Hall switches are used to determine the absolute rotor position. The Hall devices are arranged on a circuit board, which is aligned with a magnet on the rotor so that a defined relationship between the Hall outputs and the motor back EMF can be established. This solution has the disadvantage that additional room must be made for the commutation circuit board and for the encoder. In addition, Hall devices are sensitive to acoustic noise, current spikes, temperature and can be difficult to align.
Because of these disadvantages, encoders were developed which provide incremental output signals as well as commutation output signals from a single code wheel. Encoders of this type are sold, for example by the assignee of the present application. Pending patent application Ser. No. 08/980,047, U.S. Pat. No. 5,936,236 assigned to the present assignee describes such an encoder type.
The code wheel of these encoders usually has an incremental track, and at least three additional code tracks for generating the commutation output signals. This results in a minimum of four data tracks arranged concentrically on the code wheel. To provide the incremental output signals from the incremental track at least two photodetectors are necessary which are arranged in a spatial relationship so that two incremental signals are generated which have a phase shift of 90.degree. with respect to each other. For the commutation output signals, each of the code tracks is scanned by a further photodetector which provides either a HIGH output or a LOW output. The combination of the three commutation output signals allows determination of the absolute rotor position within a certain range.